1. Field of the Invention
The present invention relates to a scanning electron microscope and, more particularly, to a scanning electron microscope capable of producing high-resolution, distortion-free images when a retarding method in which a negative voltage is applied to the specimen is utilized, even if the surface of the specimen contains tilted (inclined) portions or discontinuous steps.
2. Description of Related Art
In scanning electron microscopy, specimens are imaged using an electron beam accelerated by low voltages to avoid specimen charging and damage. At this time, aberration in the objective lens tends to deteriorate as the energy of the electron beam passed through the lens decreases. To avoid this, one method has been put into practical use. In particular, this method consists of causing an electron beam having increased energy to pass through the objective lens area and applying a negative potential to the specimen to thereby decelerate the electron beam that is about to enter the specimen. This method is known as the retarding method and permits high-resolution imaging of the specimen. In this case, secondary electrons produced from the specimen are captured by the magnetic field set up by the objective lens, then pass through the inner polepiece of the objective lens, and finally travel upwardly from the lens. Therefore, the secondary electrons are detected inside or above the objective lens. In this retarding method, an electric field that decelerates the primary electron beam from the electron gun is produced between the specimen and objective lens. Where the surface of the specimen is almost flat and is not tilted (inclined relative to a plane perpendicular to the optical axis of the beam), the electric field between the specimen and objective lens maintains an axisymmetry with respect to the optical axis of the beam. Consequently, no astigmatism (off-axis astigmatism) is given to the primary electron beam.
Where the specimen surface is tilted, the axisymmetry of the electric field between the specimen and objective lens with respect to the optical axis of the electron beam is broken. A lateral electric field component is produced on the optical axis. As a result, the off-axis astigmatism in the electron beam increases and impairs the resolution. A countermeasure against this problem is disclosed, for example, in Japanese Patent Application No. 2002-189936, entitled “Scanning Electron Microscope”, where a scanning electron microscope having a cylindrical shield electrode mounted in the electron beam path between the objective lens and specimen is proposed. The shield electrode and specimen are placed at nearly the same potential.
In the retarding method described above, however, where the specimen surface has portions tilted at more than a given angle or discontinuous steps, the axisymmetry of the electric field between each of such tilted portions and discontinuous steps and the objective lens with respect to the optical axis of the electron beam is broken. This induces a lateral electric field component on the optical axis, increasing the off-axis astigmatism. As a result, the circular, sharply focused electron beam assumes a cross section of an elongated ellipse, deteriorating the resolution. Furthermore, the electron beam scanning linearly over the specimen undergoes deflection distortion, thus distorting the image.